This invention relates to integrated circuit devices and methods, and more particularly to integrated circuit fuse circuits and related methods.
Fuses are widely used in integrated circuits in order to program values or set operating parameters for the integrated circuit. In a conventional fuse, a pad is formed at both ends of the fuse, and the fuse can be directly programmed via the pads. The fuse may be programmed during testing or at other times during the manufacture of the integrated circuit. Such a conventional fuse can generally be programmed easily and accurately. Unfortunately, the need to provide a pair of pads for each fuse may limit the number of fuses that can be used in an integrated circuit, due to constraints on the number of pads that can be provided.
In order to solve this problem, it is known to provide a plurality of fuses in an integrated circuit and to program these fuses using a fusing current and a switch. More specifically, an internal switch is activated in response to an external control signal. A predetermined programming current, also referred to as a fusing current, is supplied to the appropriate fuse in response to activation of the switch.
When such programming techniques are used, however, the amount of current that may be supplied to the fuse may be limited. Accordingly, when programming the fuse, the fuse may not be opened. Rather, the impedance of the fuse may increase by an order of magnitude, for example from several tens of kxcexa9, to several hundreds of kxcexa9. This contrasts sharply with a conventional fuse which essentially is a short circuit when unprogrammed, and an open circuit when programmed.
Unfortunately, it may become more difficult to accurately detect whether such a fuse has been programmed, with a high degree of accuracy. See, for example, U.S. Pat. No. 5,731,733 to Denham entitled xe2x80x9cStatic, Low Current Sensing Circuit for Sensing the State of a Fuse Devicexe2x80x9d.
The accuracy of sensing may be increased by reducing the resistance value of the fuse. However, the reduced resistance may increase the amount of current consumption in the fuse, and thereby undesirably increase the power consumption of the integrated circuit. Accordingly, there continues to be a need for improved fuse circuits and methods that can accurately sense whether a fuse is programmed.
It is therefore an object of the present invention to provide improved fuse programming and sensing circuits and related methods.
It is another object of the present invention to provide improved fuse sensing circuits that can sense whether or not a fuse is programmed, even though programming the fuse only increases the impedance thereof, but does not open circuit the fuse.
These and other objects are provided, according to the present invention, by fuse circuits and methods that program a fuse by applying a program current to the fuse that is sufficient to increase the impedance of the fuse without opening the fuse, that apply a sensing current to the fuse in response to a reset signal and that otherwise sense whether the fuse is programmed without applying sensing current to the fuse. By maintaining essentially zero current through the fuse after programming, reduced current consumption and accurate sensing of programming state can be maintained, even though programming the fuse does not produce an open circuit, but rather only increases the impedance of the fuse.
More specifically, according to the invention, a sensing current is applied to the fuse, to thereby determine the programmed or unprogrammed state of the fuse, wherein the fuse has a low impedance in the unprogrammed state, and the fuse has a high impedance but is not open in the programmed state. The programmed or unprogrammed state of the fuse is latched. Application of the sensing current is then terminated. The sensing current may be applied in response to activation of a reset signal, and application of the sensing current may be terminated in response to deactivation of the reset signal.
Even more specifically, fusing circuits according to the invention include a fuse and a first circuit that programs the fuse by applying a programming current to the fuse that is sufficient to increase the impedance of the fuse without opening the fuse. A current supply, a first switch and a second switch are serially connected to the fuse. The first switch and the second switch define a node therebetween. A second circuit generates a first control signal for the first switch and a second control signal for the second switch in response to a reset signal. Finally, a third circuit produces an output signal that indicates whether the fuse is programmed in response to a voltage in the node.
The first and second switches preferably comprise respective first and second field effect transistors of opposite conductivity types. The third circuit preferably comprises an inverter that produces the output signal from the voltage of the node. The second circuit is preferably responsive to the output signal and the reset signal. The second circuit preferably comprises a first logic circuit that is responsive to the reset signal and to the output signal, to generate the first control signal and a second logic circuit that is responsive to the reset signal and to the output signal to generate the second control signal. Accordingly, current may be applied to the fuse only during application of the reset signal. Otherwise, programming of the fuse is sensed without applying sensing current to the fuse. Low power and reliable sensing can therefore be obtained.